Quarantine Plants Before Bringing Them Inside for Winter

Before you move a potted plant indoors for winter, quarantine it first: inspect the foliage, the soil and the pot, wash it down, and keep it away from your other houseplants for three weeks. 

Why quarantine before bringing plants in for winter?

A pot that spent the summer outside almost always arrives with insects already on it. The trouble is that those insects stay hidden until the plant is standing beside your indoor collection, where there is no rain to knock them off and no predator working the population down. Indoors, warm and dry, a few spider mites on one plant can become an infestation across every plant in the room.

"Get rid of pests on the plant or in the soil before taking plants inside, and then quarantine the plants away from the indoor plants."
- Ask Extension, Bringing plants indoors for winter #920266

What to inspect before the move: leaves, soil, pot and drainage holes

Clemson Cooperative Extension lists the hidden places most people miss: check the foliage on the top and underside of every leaf, the soil surface, under the rim of the pot, and the bottom of the pot, and take the plant out of its pot to look through the drainage holes.

Look for the common outdoors carry-overs: aphids, spider mites, whiteflies, scale insects, earwigs and ants. Spider mites are the recurring indoor problem, especially on herbs like rosemary, and they hide on the undersides of the needles where a quick glance will not spot them. University of New Hampshire Extension notes a heavy stream of water sprayed on the foliage every day or so knocks mites off the leaves.

How to wash a plant before bringing it inside

Once you have inspected, wash the plant down with a strong spray of water over the foliage, top and underside, and flush the pot if anything is living in the soil. For plants that sat on the ground, clean the outside of the pot as well. Clemson Cooperative Extension recommends spraying the entire plant including the soil and the pot with insecticidal soap to treat what the water does not remove. No nutrition product replaces this step; washing and treatment are the pest control, and a fertilizer cannot do it.

How long to quarantine houseplants before winter?

Quarantine the washed plant away from the rest of your indoor collection for three full weeks. The three weeks is the part people skip, and it is the part that does the work: isolation only helps if the plant stays separated for the entire window, so a pot parked beside your other houseplants is not in quarantine. Per Ask Extension, after three weeks with no sign that insects survived, the plant can be introduced to the plants that stay indoors.

Quarantine checklist for bringing plants indoors

• Inspect leaves top and bottom, soil surface, pot rim, pot bottom and drainage holes.
• Wash the foliage with a strong spray of water; flush the pot if insects are in the soil.
• Treat with insecticidal soap if you find pests, following the label directions.
• Keep the plant alone, away from other houseplants, for three full weeks.
• After three weeks with no pests, introduce it to the collection.
  

Bringing plants in for winter: the cold-night trigger

Move plants in ahead of the first cold nights of autumn, before temperatures in your region drop toward freezing. The quarantine science does not depend on a single temperature number, so follow your local frost forecast and your plant's cold sensitivity rather than one universal threshold. Do the inspection, the wash and the three-week isolation, and your indoor collection stays clean all winter.